This invention relates to a power wrench of the type having a torque responsive release clutch.
During tightening of screw joints a power wrench of this type is brought to deliver full power until the desired torque level is reached and the torque responsive clutch releases. Then, the power supply to the power wrench motor is interrupted, manually or automatically, whereby the motor is stopped. During the very last revolutions of the power wrench motor, the speed of relative rotation between the driving and the driven halves of the release clutch becomes slow enough to provide for reengagement of the clutch.
As the clutch finally reengages an undersirable jerk arises in the power wrench as the inertia of the motor rotor and the driving clutch half is transferred to the driven clutch half. Such a jerk is not only very annoying for the operator of hand held tools but it will also give rise to an undesirable torque addition in the screw joint being tightened.
These problems are solved by employing in power wrenches of the above type an inactivatable torque non-responsive clutch which is connected in series with the release clutch in order to completely disconnect the motor rotor from the power wrench output shaft as the desired torque level is reached.
In one prior art power wrench of this type the torque non-responsive clutch is located between the motor and the release clutch, which means that the inertia of the entire release clutch, after reengagement, is rotatively coupled to the output shaft. This is a serious drawback in that it results in a large mean shift, e.g. a big difference in final tightening torque between hard and soft joints.
In another prior art power wrench of this type, the torque non-responsive clutch is arranged to be shifted to its inactive position by one of the release clutch halves during disengagement of the release clutch. Such an arrangement is disadvantageous in that the torque non-responsive clutch is forced out of engagement under full torque load which gives rise to considerable frictional losses which in turn will cause the torque responsive clutch to release at an increased torque level.
The present invention intends to solve the above problems, which is accomplished by the power wrench defined in the claims.